Conventionally, this type of variable wavelength optical filter module has been known to have the following structure. That is, a variable wavelength optical filter is used which can transmit a ray of light having a specific wavelength, via interference action when an incident light is subjected to multiple reflection between a pair of reflecting plates. Then, a small clearance between the pair of the reflecting plates of the variable wavelength optical filter is displaced to vary the wavelength of the light being transmitted through it. Then, the transmitted light is detected with a detector.
Examples of application of conventional variable wavelength optical filter modules include: multi-gas sensors for sensing kinds of gases by using mid-infrared rays, and sensors for sensing urine glucose levels or a blood glucose levels by using near-infrared rays.
Note that, in the conventional variable wavelength optical filter module, there are arranged a light source to emit a ray of light, the variable wavelength optical filter, and the detector, on a straight line along the optical axis of the light source.
Note that prior art documents relating to the invention of the present application have been known, including Patent Literature 1, for example.